0.18 to 0.19 migration guide missing entry: `ImageNode` now draws in the content box by default

The 0.18 to 0.19 migration guide has no entry for the `ImageNode` rendering change introduced in bevyengine/bevy#24154

0.19 added `ImageNode::visual_box` (`VisualBox`), defaulting to `VisualBox::ContentBox`. In 0.18, an `ImageNode`'s image was drawn over the node's full bounds; in 0.19 it is drawn only within the content box — i.e. inset by the node's `border` and `padding`.

Any node combining an `ImageNode` with nonzero `padding` (or `border`) renders differently after upgrading, with no compile error or warning. I ran into this with the exact use case that motivated bevyengine/bevy#24150: a 9-sliced button background with padding used to inset a text label. After upgrading, the sliced background shrinks to hug the label and the padding appears to no longer work.

Start with the 0.18 to 0.19 migration guide and review its existing entries for the appropriate placement and style. Document the ImageNode visual-box default change and its effect on padding and borders, then confirm the new entry explains the upgrade impact described in the issue.

Written by the indexing model from the issue text.